The present invention relates to a process and a plant for the continuous hot dip-coating of a metal strip, especially a steel strip.
In many industrial applications, steel sheet is used which is coated with a protective layer, for example for corrosion protection, and usually coated with a zinc layer.
This type of sheet is used in various industries to produce all kinds of parts, in particular visual parts.
To obtain this kind of sheet, continuous dip-coating plants are used in which a steel strip is immersed in a bath of molten metal, for example zinc, which may contain other chemical elements, such as aluminium and iron, and possible additional elements such as, for example, lead, antimony, etc. The temperature of the bath depends on the nature of the metal, and in the case of zinc the temperature of the bath is around 460° C.
In the particular case of hot galvanising, as the steel strip runs through the molten zinc bath, an Fe—Zn—Al intermetallic alloy with a thickness of a few tens of nanometres forms on the surface of the said strip.
The corrosion resistance of the parts thus coated is provided by the zinc, the thickness of which is controlled usually by air wiping. The adhesion of the zinc to the steel strip is provided by the layer of the aforementioned intermetallic alloy.
Before the steel strip passes through the molten metal bath, this steel strip firstly runs through an annealing furnace in a reducing atmosphere where the purpose is to recrystallise it after the substantial work hardening resulting from the cold-rolling operation and to prepare its surface chemical state so as to favour the chemical reactions necessary for the actual dip-coating operation. The steel strip is heated to about 650 to 900° C. depending on the grade, for the time needed for recrytallisation and surface preparation. It is then cooled to a temperature close to that of the bath of molten metal by means of heat exchangers.
After it has passed through the annealing furnace, the steel strip runs through a duct, also called a “snout”, containing an atmosphere which protects the steel, and is immersed in the bath of molten metal.
The lower part of the duct is immersed in the bath of metal in order to define, with the surface of the said bath and inside this duct, a liquid seal through which the steel sheet passes as it runs through the said duct.
The steel strip is deflected by a roller immersed in the metal bath. It emerges from this metal bath and then passes through wiping means used to regulate the thickness of the liquid metal coating on this steel strip.
In the particular case of hot galvanising, the surface of the liquid seal inside the duct is generally covered with zinc oxide, coming from the reaction between the atmosphere inside this duct and the zinc of the liquid seal, and with solid dross coming from the steel strip dissolution reaction.
These dross or other particles, in supersaturation in the zinc bath, have a density less than that of the liquid zinc and rise to the surface of the bath and especially to the surface of the liquid seal.
The running of the steel strip through the surface of the liquid seal causes entrainment of the stagnant particles. These particles entrained by the movement of the liquid seal, which depends on the speed of the steel strip, are not removed from the volume of the bath and emerge in the region where the strip is extracted, creating visual defects.
Thus, the coated steel strip has visual defects which are magnified or revealed during the zinc wiping operation.
This is because the foreign particles are retained by the air wiping jets before the said particles are ejected or broken up, thus creating streaks of lesser thickness in the liquid zinc having a length ranging from a few millimeters to a few centimeters.
Various solutions have been proposed to try to remove the zinc particles and the dross from the surface of the liquid seal.
A first solution for avoiding these drawbacks consists in cleaning the surface of the liquid seal by pumping off the zinc oxides and dross coming from the bath.
These pumping operations allow the surface of the liquid seal to be cleaned only very locally at the point of pumping and their effectiveness and range of action are very low, which does not guarantee that the liquid seal through which the steel strip passes is completely cleaned.
A second solution consists in reducing the area of the liquid seal at the point through which the steel strip passes by placing a sheet-metal or ceramic plate at this liquid seal in order to keep some of the particles present at the surface away from the strip and to achieve self-cleaning of the liquid seal by this strip.
This arrangement does not keep away all the particles present at the surface of the liquid seal and the self-cleaning action is greater the smaller the area of the liquid seal, this being incompatible with industrial operating conditions.
Furthermore, after a given operating time, the store of particles outside the plate becomes greater and greater and clusters of particles end up being detached and coming back onto the steel strip.
The addition of a plate emerging at the surface of the liquid seal also forms a preferential site for trapping zinc dust.
Another solution consists in adding a frame to the surface of the liquid seal in the duct and surrounding the steel strip.
This arrangement does not make it possible to remove all the defects associated with the entrainment of zinc oxides and dross caused by the running of the steel strip.
This is because the zinc vapour at the liquid seal will condense on the walls of the frame and at the slightest disturbance, brought about by the vibrations or thermal inhomogeneities of the immersed strip, the walls of the frame become fouled and thus become regions of retention of foreign matter.
This solution can therefore operate only for a few hours, at best a few days, before itself becoming an additional cause of defects.
Thus, this solution deals only partly with the liquid seal and does not make it possible to achieve a very low defect density satisfying the requirements of customers desiring surfaces free of visual defects.
Also known is a solution which aims to clean the liquid seal by replenishing the bath of molten metal.
The replenishment is achieved by introducing pumped liquid zinc into the bath near the region where the steel sheet is immersed.
There are great difficulties in implementing this solution.
This is because it requires an extremely high pumping rate in order to provide an overflow effect and the pumped zinc injected at the liquid seal contains dross generated in the zinc bath.
Moreover, the pipe for replenishing the liquid zinc may cause scratches on the steel strip before it is immersed and is itself a source of defects caused by the accumulation of condensed zinc vapours above the liquid seal.
Also known is a process based on the replenishment of zinc at the liquid seal and in which this replenishment is accomplished by means of a stainless steel box surrounding the steel strip and emerging at the surface of the liquid seal. A pump sucks off the particles entrained by the overflow thus created and delivers them into the volume of the bath.
This process also requires a very high pumping rate in order to maintain a permanent overflow effect insofar as the box surrounding the strip in the volume of the bath above the bottom roller cannot be hermetically sealed.
The object of the invention is to provide a process and a plant for the continuous galvanising of a metal strip which make it possible to avoid the abovementioned drawbacks and to achieve the very low density of defects meeting the requirements of customers desiring surfaces free of visual defects.